This invention relates to a novel method for molding a heated piece of glass into glass products, in particular optical lenses, having high surface accuracy and thickness accuracy.
For obtaining optical elements made of glass, there has been practiced a method which, replacing an old, traditional method depending upon grinding, is directed to obtaining a molded glass product directly by preparing a glass piece for each piece of a desired optical element and pressing this glass piece between mold surfaces of a pair of molds. This prior art method, however, has the disadvantage that the molding step is not sufficient for achieving desired surface accuracy and surface quality and desired accuracy in dimensions including thickness of lens and, accordingly, cumbersome subsequent operations including coarse abrasion and precision grinding are required with a result that extra time and labor must be spent.
Various techniques have been proposed and practiced for obviating the above described operations subsequent to the molding. It is desired that finished products made according to these techniques generally should have surface quality, i.e., surface roughness, of within .+-.0.02 .mu.m, surface accuracy of within six Newton's rings, irregularity of within .lambda./2 and thickness accuracy of within about .+-.50 .mu.m.
For satisfying these requirements, considerations should be given to such factors as glass composition suitable for this type of molding, metering of a glass piece to be molded with a suitable amount of surplus taken into account, dimensions and shape, surface quality and other qualities, a heat treatment of the glass piece as a pretreatment and temperature distribution in the glass piece immediately before molding, a method for supporting the glass piece, material used for a molding surface of a mold, surface quality, surface accuracy and a dimensional design with contraction after molding taken into account, temperature of the mold, accurate symmetrical positioning of the molds, timing of contact and fitting of the molds against the glass piece, pressure of molding by the molds and viscosity or value of plastic flow and volume resulting from fall in the temperature of the glass piece or change in dimensions due to contraction in the pressing direction and the moving speed and distance of displacement and stop of moving of the molds corresponding to such change in dimensions and accurate control of relative position of and interval between the molds, and time of holding the molded glass in the molds.
Among these factors to be considered, the moving speed of the mold in the final stage of molding of the glass piece is, in the case of normal optical lenses, 0.5.about.2 .mu.m per second in the fastest case and the distance of displacement is several .mu.m to several tens .mu.m in terms of the entire moving distance. Accuracy in controlling of the moving speed and distance of displacement, however, greatly affects the accuracy of the optical element.
It is an object of the invention to provide a novel method capable of accurately controlling the moving speed and distance of displacement of the mold among the above described various factors to be considered. In one aspect of the invention, there is also provided a method for accurately controlling the timing of stopping the movement of the mold in the molding step.
For achieving the above described object of the present invention, these is provided a method for molding high-precision glass products by heating and softening a piece of glass at least in the surface portion thereof and molding the glass piece by pressing the same between molding surfaces of a pair of molds comprising the step of causing relative movement of the molds toward each other during the pressing to follow change in dimension of a heated interval control member provided between said pair of molds, said interval control member having a coefficient of thermal expansion which is equal to or larger than that of the glass piece and therefore being capable of thermal contraction in the pressing direction by an amount which is substantially equivalent to or larger than that of the glass piece in the pressing direction due to cooling of the glass piece, and said change in dimension of said interval control member being caused by thermal contraction due to cooling thereof.
According to the invention, movement of the mold during pressing is controlled by an interval control member which is provided as a different member from the molds but is in intimate contact with the molds. The material of this interval control member is selected from materials having a coefficient of thermal expansion which is equal to or larger than the coefficient of thermal expansion of the glass piece to be pressed (temperature range of measurement being below a strain point) and the relative movement of the molds during pressing therefore follows change in dimension caused by thermal contraction of the interval control member (i.e., decrease in the dimension of the interval control member which is equivalent to or larger than decrease in the thickness of the glass piece in the pressing direction caused by thermal contraction) whereby pressure of the molding surfaces of the molds against the glass piece and the moving speed of the molds can be accurately controlled.
According to another aspect of the invention, there is provided a method for molding high-precision glass products as defined in claim 1 further comprising the step of detecting, when an interval between the opposing molding surface of said pair of molds during pressing of the glass piece has been reduced to a value at which the relative movement of the molds should be terminated, a predetermined temperature corresponding to said value of said interval control member which has been contracting with the accompanying relative movement of the molds and transmitting a detection signal to a control system controlling the movement of the molds to terminate the relative movement of the molds.
According to the above described aspect of the invention, temperature of the interval control member is detected at a time point when the interval between a pair of molds has reached a predetermined distance following decrease in the dimension of the interval control member to a predetermined value due to thermal contraction caused by fall in the temperature, and a detection signal is transmitted to a mold movement control system to terminate the movement of the mold whereby the interval between the molding surfaces of the molds in the final stage of pressing is accurately controlled.